This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Patent Application 2000-161249 filed on May 30, 2000, the entire content of which is incorporated herein by reference.
This invention generally relates to a hydraulic brake device. More particularly, the present invention pertains to hydraulic brake device in a vehicle that is provided with an assisting device for assisting master cylinder actuation in response to brake pedal operation.
A known negative pressure booster (servo motor) for use in a power braking system is disclosed in a U.S. Pat. No. 3,910,048 issued in 1975. The servo motor includes a pressure ratio changer in which a first piston and a second piston supply a master cylinder with an operative force. The first piston is connected with a wall within the servo motor which is moved by a pressure differential across the wall. The second piston is concentrically positioned within the first piston to transmit an input force which operates a control valve to produce the pressure differential and also operates a hydraulic lock valve located within a master cylinder. As the first piston and the second piston move together in response to movement of the wall, hydraulic fluid flows into a locking chamber past the lock valve. When the maximum force output capable of being generated by the pressure differential is reached, the simultaneous movement of the first and the second pistons will cease. Further manual force input from the operator will move the second piston within the first piston to close the lock valve and hold the hydraulic fluid within the locking chamber to prevent the first piston from moving.
More specifically, as the first and the second pistons move forward, hydraulic pressure develops in pressure chambers of the master cylinder. The output from the first and the second pistons follows a line (line 188 shown in FIG. 3 of U.S. Pat. No. 3,910,048) until the entire second chamber in the servo motor contains air at atmospheric pressure. At a particular point (point 190 in FIG. 3 of the aforementioned patent), the input force from the pedal is transmitted through a plunger and a sleeve into the second piston (shown in FIG. 1 of the aforementioned patent). This input force causes the second piston to independently move and permits a valve spring to close a hydraulic passage. As the second piston moves further, the output will follows a line (line 192 shown in FIG. 3 of the patent). In the event a negative pressure is unavailable at the intake manifold, an input force applied to the brake pedal initially moves the second piston within the first piston to permit immediate closure of the hydraulic passage by the lock valve. The output from the second piston follows a line (line 196 shown in FIG. 3 of the patent). This line exhibits a higher output than the line 195 where both pistons are moved during a no power condition.
The servo motor for use in a power braking system disclosed in the aforementioned patent is configured to provide an output following the line 192 in FIG. 3 of the patent by enclosing the increased hydraulic pressure in the locking chamber in response to the movement of the second piston within the first piston after reaching the point 190. Even when the servo motor fails (e.g., a negative pressure is unavailable), the output can follow the line 196. To provide an output following the line, as shown in FIG. 1 of the patent, the servo motor serving as an assisting device needs to be configured for assuring a sufficient relative amount of movement of the second piston relative to the first piston. Accordingly, the structure of the known servo motor requires substantial change and the increase of the cost is unavoidable. Similarly, when the braking force is further increased after the assisting limit by the servo motor, the structure of the known servo motor still needs to be changed.
A need thus exists for an improved hydraulic brake device for a vehicle having an assisting device that drives a master cylinder in response to a brake pedal operation.
It would thus be desirable to provide a hydraulic brake device for a vehicle that is able to assure a proper input-output performance not only when the assisting device such as a brake booster fails, but also after reaching the assisting limit, with minimal structural changes in the assisting device.
According to one aspect of the present invention, the hydraulic brake device for a vehicle includes a master cylinder having a master piston for supplying brake pressure to a wheel cylinder in response to a brake pedal depression, an assisting device for assisting the actuation of the master piston in response to the brake pedal depression, and an auxiliary piston provided rearward of the master piston and including a large diameter portion having an effective cross-sectional area larger than that of the master piston and a small diameter portion provided in front of the large diameter portion and having an effective cross-sectional area smaller than that of the master piston. The auxiliary piston is operatively associated with the assisting device. A first pressure transmitting chamber is provided between the large diameter portion and the master piston, and a second pressure transmitting chamber is provided between the small diameter portion and the master piston. A first valve mechanism hydraulically closes the first pressure transmitting chamber when the actuation of the master piston is assisted by the assisting device through the auxiliary piston and establishes hydraulic communication between the first pressure transmitting chamber and a reservoir of the master cylinder when the master piston is not actuated by the assisting device. A second valve mechanism hydraulically connects the second pressure transmitting chamber with the first pressure transmitting chamber to increase the brake pressure when the actuation of the master piston is assisted by the assisting device through the auxiliary piston, hydraulically closes the second pressure transmitting chamber after the pressure in the first pressure transmitting chamber reaches a predetermined value, and hydraulically connects the second pressure transmitting chamber with the first pressure transmitting chamber when the master piston is not actuated by the assisting device. The predetermined pressure is preferably set to the pressure immediately before the assisting limit by the assisting device.
When the master piston is assisted through the auxiliary piston by the assisting device, the first pressure transmitting chamber is hydraulically sealed by the first valve mechanism. Then the auxiliary piston and the master piston are hydraulically connected. The brake pressure is increased by connecting the second pressure transmitting chamber with the first pressure transmitting chamber by the second valve device. After the pressure in the first transmitting chamber reaches the predetermined pressure, the second pressure transmitting chamber can be hydraulically sealed. When the assisting is not carried out by the assisting device, the pressure in the first pressure transmitting chamber becomes the atmospheric pressure by being connected with the reservoir by the first valve mechanism. The pressure in the second pressure transmitting chamber is decreased to atmospheric pressure by being connected with the first transmitting chamber by the second valve mechanism. Accordingly, the auxiliary piston and the master piston can be mechanically connected.
The hydraulic brake device further includes a first hydraulic passage formed in the auxiliary piston for connecting the first pressure transmitting chamber with the reservoir. The first valve mechanism includes a first valve seat disposed in the first hydraulic passage, a first valve body adapted to be seated on or separated from the first valve seat for closing or opening the first hydraulic passage, a first biasing means always biasing the first valve body toward the first valve seat, and a brake input transmitting member for driving the first valve body in response to the brake pedal depression when the assisting device is not operated.
The hydraulic brake device also includes a second hydraulic passage formed in the auxiliary piston for connecting the second pressure transmitting chamber with the first pressure transmitting chamber. The second valve mechanism includes a second valve seat disposed in the second hydraulic passage, a second valve body to be seated on or separated from the second valve seat for closing or opening the second hydraulic passage, a second biasing means always biasing the second valve body toward the second valve seat, a pressure responsive member for moving the second valve body to be separated from the second valve seat in response to the pressure in the first pressure transmitting chamber, and a third biasing means for always biasing the pressure responsive valve toward the second valve body with a biasing force larger than that of the second biasing means.
The assisting device can be in the form of a negative pressure booster serving as a negative pressure assisting device or a hydraulic pressure booster serving as a hydraulic pressure assisting device. Elastic members are provided between the auxiliary piston or the master piston and the assisting device and the brake input transmitting member is desirably arranged to connect with the brake pedal without being in contact with the elastic members.
According to another aspect of the invention, a hydraulic brake device for a vehicle includes a master cylinder with a master piston for supplying brake pressure to a wheel cylinder in response to depression of a brake pedal, an assisting device for assisting actuation of the master piston in response to the depression of the brake pedal, and an auxiliary piston provided rearward of the master piston and including a larger diameter portion and a smaller diameter portion provided in front of the larger diameter portion. The auxiliary piston is operatively associated with the assisting operation of the assisting device. A first pressure transmitting chamber is located between the larger diameter portion and the master piston, and a second pressure transmitting chamber is located between the smaller diameter portion and the master piston. A first valve mechanism hydraulically closes the first pressure transmitting chamber when actuation of the master piston is assisted by the assisting device through the auxiliary piston and establishes hydraulic communication between the first pressure transmitting chamber and a reservoir when the master piston is not actuated by the assisting device. A second valve mechanism hydraulically connects the second pressure transmitting chamber with the first pressure transmitting chamber when the master piston is not actuated by the assisting device.
In accordance with another aspect of the invention, a hydraulic brake device for a vehicle includes a master cylinder having a master piston for supplying brake pressure to a wheel cylinder in response to depression of a brake pedal, an assisting device for assisting actuation of the master piston in response to the depression of the brake pedal, an auxiliary piston provided rearward of the master piston and including a larger diameter portion and a smaller diameter portion provided in front of the larger diameter portion, with the auxiliary piston being operatively associated with assisting operation of the assisting device, a first pressure transmitting chamber between the larger diameter portion and the master piston, and a second pressure transmitting chamber between the smaller diameter portion and the master piston. A first valve mechanism hydraulically closes the first pressure transmitting chamber when actuation of the master piston is assisted by the assisting device through the auxiliary piston and establishes hydraulic communication between the first pressure transmitting chamber and a reservoir when the master piston is not actuated by the assisting device. A hydraulic passage formed in the auxiliary piston connects the second pressure transmitting chamber with the first pressure transmitting chamber. A valve seat is disposed in the hydraulic passage, a valve body is adapted to be alternatively seated on and separated from the valve seat for alternatively closing and opening the hydraulic passage, and a spring always biases the valve body toward the valve seat.